The present invention relates to a method of measuring a dimension, such as the thickness of thick metallurgical products such as blooms, girders, and sections for example.
The thickness of thin metallurgical products, such as sheet or strip, is frequently measured by means of X-rays or gamma rays according to methods mainly based on absorption phenomena in the products to be measured.
As the thickness of the products to be measured increases, so also the absorption of X-rays and gamma rays increases. This results in the need to use more powerful rays on thicker products. Consequently, one is faced with a higher cost due to increased energy consumption and a certain amount of risk in view of the nature and the power of the rays used.
Moreover, it should be noted that the methods based on such absorption phenomena permit only the solid part of the products to be measured . Accordingly, when applied to pipes these methods permit the measurement of the thickness of the walls only and not the outside diameter. The same kind of difficulties is faced with Tee or channel sections or girders and in general with any products having hollow portions.
Furthermore, the results of measurements made in accordance with methods are influenced by the density of the product whose thickness is to be determined. This results in the necessity of applying such methods to, products which are as homogeneous as possible, with no internal faults such as blowholes and inclusions.
These circumstances tend to reduce the field of application of X-rays or gamma rays, which are found to be of practical interest only for measuring the thickness of thin materials. To remedy these inconveniences, in U.S. Pat. No. 3,619,070 I have already described a method in which on each side of the body whose thickness is to be measured one point is selected, the points being situated substantially on a common normal to the median plane of the body, the two points being such that the planes tangential to the sides of the body at these points are substantially parallel; a beam of optical rays is directed towards each of these two points and a high proportion of the rays reflected from each of the points is picked up in the form of two distincts beams, all the rays forming each of these beams are made parallel to each other, these two beams are then orientated towards a common plane, and the distance between the images resulting from the intersection of these beams with the said plane is measured, this distance being representative of the thickness to be measured.
The distance between the images resulting from the intersection of the beams from the body whose thickness is to be measured with the plane towards which they are oriented, may be measured in a number of ways. For example, this distance is measured by either locating a graduated reticle in the image plane, or causing such images to appear on a graduated screen, or even causing them to appear on a television screen subjected to electronic scanning.
The results thus obtained have been found to be quite satisfactory. However, in the case of a television screen with electronic scanning, one is using relatively expensive equipment whose performance is above all limited owing to lack of scanning linearity which results in a loss of accuracy.